Description of the Prior Art
During the manufacture of integrated circuits, vacuum equipment is generally used to process semiconductor wafers. Vacuum processing equipment, such as ion implanters, etchers, sputter coaters and chemical vapor deposition machines, are usually employed. During the processing and handling of the wafers along the production line, defects on the surface of the wafer can appear due to various conditions that occur in the equipment. For example, metal material may adhere to the wall of the sputter coater, the metal material can flake off the wall, and the flaked metal particles can land on the wafers. Any dust particles or contaminants that appear in the processing equipment tend to adhere to the wafer surface. As a result of the appearance of such undesirable particles on the wafer surface, short circuits or breaks in surface patterns are created which seriously degrade the performance of the finished integrated circuit.
It is highly desirable to detect such surface defects and to localize and control the sources of the problem in order to obtain optimum yields in the production of contamination-sensitive devices such as integrated circuits.
Various types of surface defects scanners have been described in the prior art. One type of surface scanner inspects surfaces of unpatterned wafers by using a focused laser beam that is scanned back and forth, so that the focus of the beam follows an arc. The wafer is moved in a direction perpendicular to the plane of the arc so that its surface is raster scanned. In such a system, which is described in the textbook "Silicon Processing For The VLSI Era, Volume I: Process Technology", by S. Wolf and R. N. Tauber, published by the Lattice Press, 1986, on page 514, seq. et., the moment arm of the arc is shown to be very long in order maintain the wafer surface within the depth of focus of the laser beam. The length of the moment arm causes the beam to be approximately normally incident to the wafer surface. Since the surface of the wafer is very flat, the laser beam will undergo specular reflection. If a surface defect is present, the reflected beam will scatter diffusely and the scattered light will be gathered by an optical collector and will be detected by a photomultiplier.
Another type of surface defect scanner employs a mercury-xenon arc lap to provide intense illumination over the entire surface of the wafer. A video camera that is off the axis of the light source will not see the light that is specularly reflected from the wafer surface, but will see light that is scattered from small defects.
Prior art surface defect scanners that use laser beams are characterized by depth of field problems. The prior art scanners employ a scanning light beam which traces an arc at its focal point. If the scanning beam which traces the arc is in a plane normal to the plane of the wafer surface, i.e., the angle of incidence .beta. equals 90.degree., then the focal point does not stay in the plane of the wafer surface but moves above and below the wafer surface. Therefore, the intensity of the light at the wafer surface will vary, because the sensitivity of the surface scanner is a function of the intensity of the light at the wafer surface. Prior art systems attempt to solve this problem by using a very long focal length which minimizes this undesirable deviation but provides a larger depth of field of the beam. As a result, the scanner system is made to be very large which in effect precludes its use in vacuum chambers with limited space.
The prior art systems are extremely large and bulky because there is a need to use a long focal length so that the arc traced by the scanning light is almost linear. In the system that employs a laser surface defects scanner, a scanning mirror is required to be positioned a large distance above the surface of the wafer. The large distance is determined by the requirement for the wafer surface to lie within the depth of focus of the laser spot across the full arc of the scan. The large depth of field thus necessitates a relatively large vacuum chamber.
Furthermore, in the system that uses an arc lamp, the apparatus must be large enough to accommodate the lamp and the lenses that are required to illuminate the wafer surface uniformly, and the viewing camera. Therefore, in view of the large size of the scanner equipment and incompatibility with vacuum chambers, prior art systems used for surface defect detection have not been installed in vacuum processing equipment.